Overrun Clutch Air Bleed Checkball
WARNING: This page is about a different car, the 2010 GMC Sierra and 2010 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
This ball and capsule is located in the overrun clutch fluid circuit in the oil pump. It allows air to exhaust from the circuit as fluid pressure increases and also allows air into the circuit to displace the fluid when the clutch releases.